Ford Repair: 73 ford mustang, mustang mach1, ford mustang

Question
hello and thank you for your time
my question is this i have a 73 mustang mach1 with a 351c i just put a new batt. in it and now if i let it sit for 2 days or more and try to start it all i get is a click but if i let it sit over night it starts right up i installed a new starter relay now heres the fun part if after 2 days i try to start it i get a click i test the batt and its at 12.5 fully charged but if i put a jump on it with a batt charger it starts right up i cant think of any thing that would make a car need a jump on a fully charged batt to make it start but for some reason thats what it takes if i let it sit for 2 days or more even thou the batt is fully charged allready i am so lost can you help???

Answer
hey there, i dont understand the problem either, but i have seen a couple of time a loose connection at the battery terminals, when you jump it you are just mearly helping the  connection, see is there is a loose connection or possible just a bad connection, remove the terminals and celan and tighten everything and let me know what happenes, good luck
Chris